1. Field of the Invention
The present invention relates to a thermoelectric material having carbon nanotubes partly or fully embedded inside formed by mechanically pulverizing, mixing, and treating by heating mixed powder manufactured by a chemical reaction after mixing a first solution in which carbon nanotubes are dispersed and a second solution containing metallic salts and a method for fabricating the same.
2. Description of the Related Art
The thermoelectric material is an energy converting material wherein electric energy is generated when making a temperature difference between both ends of the material, and conversely, a temperature difference between both ends of the material is made when electric energy is given to the material.
Since the late 1930's, after thermoelectric phenomena such as Seebeck effect, Peltier effect, Thomson effect, etc. had been discovered, such thermoelectric materials have been developed as thermoelectric materials having high thermoelectric figure-of-merit along with the development of semiconductors. These thermoelectric materials are used as special power supply devices for backwoods, space, military, etc. using thermoelectricity generation, and are used for precise temperature control in semiconductor laser diodes, infrared detectors, etc. In addition, these thermoelectric materials are used as compact coolers related to computers, coolers for optical communication lasers, coolers for hot and cold water dispensers, semiconductor temperature regulating devices, heat exchangers, etc. using thermoelectric cooling.
To improve thermoelectric figure-of-merit of such thermoelectric materials, dimensionless figure-of-merit, value of ZT=(σα2/κ)T should be improved (α: Seebeck coefficient, σ: electric conductivity, κ: thermal conductivity, T: absolute temperature).
High figure-of-merit of thermoelectric materials means high energy conversion efficiency, and to increase such figure-of-merit, it is required to increase electric conductivity and Seebeck coefficient or decrease thermal conductivity.
Generally, the electric conductivity and thermal conductivity of materials have characteristics depending on each other. That is, materials having low electric conductivities are known as ones having low thermal conductivities.
However, in case of thermoelectric materials, appropriate combination of high electric conductivity and low thermal conductivity is required as verified in the above figure-of-merit ZT. Since out of the properties affecting the figure-of-merit, the Seebeck coefficient and electric conductivity mainly depend on charges, and the thermal conductivity mainly depends on phonons, it is necessary to independently control thermal and electrical properties by control of microstructure considering the same.
More particularly, the figure-of-merit can be improved by inducing increase of charge mobility and charge density in thermoelectric materials and increase of phonons-scattering in the lattice forming thermoelectric materials.
In order to achieve good combination between thermal and electric properties, research for fabricating nanostructured thermoelectric materials with nano-sized grains or thermoelectric composites to which nanophases as dispersoids are added is recently in active progress to make thermoelectric materials having high figure-of-merit.
Nanostructured thermoelectric materials can control electric conductivity and thermal conductivity of materials independently through a characteristic that nano-grained materials having certain grain sizes maximize the scattering of phonons but make charges pass through.
In the same context, making thermoelectric materials into composites is known as a method for improving thermoelectric figure-of-merit in a way that nano-dispersoids form newly large interfaces causing more scattering of lattice-phonons resulting in reduction of thermal conductivity. As a result of this, figure-of-merit can be improved due to effectively reduced thermal conductivity of the composites.
Especially, since the performance of the thermoelectric composites can be controlled depending on kinds and sizes of nano-dispersoids, thermal and electric properties of thermoelectric materials can advantageously be controlled at the same time.
However, in most of the processes for fabricating the existing thermoelectric composites, metallic oxides are used as nano-dispersoids as disclosed in Korea laid-open patent No. 2001-0028909 and Korea Patent application No. 10-2008-0087859.
Accordingly, the scattering of phonons by nano-dispersoids of metallic oxides is effectively made and the electric conductivity is maintained or slightly decreased when nano-dispersoids are dispersed in size under 10 nm. However, it is not easy to completely remove agglomeration of nano-dispersoids under dozens of nanometers only by a mechanical milling process.
In L. Zhao et al., Journal of Alloys and Compounds 455 (2008) p 259-264, nano-dispersoids, metallic oxides, and SiC, which is a nonconductive ceramic particle, are used, but there is a problem in that the electric conductivity decreases because of the nonconductive ceramic particles.
Since such problems of agglomeration of nano-dispersoids and their low conductivity eventually make it difficult to effectively use dispersoids in thermoelectric matrix phases, it becomes hard to secure synergistic effect of realizing at the same time effects of nano-structuring and nano-dispersoids through composites.